I strongly suggest using Date Increment function with tasks like this. Below is an example with two different dates:
Names Default To Here(1);
date1 = Date DMY(15,8,2021);
date2 = Date DMY(1,9,2021);
Show(Date Increment(date1, "month", -12, "start")); //Date Increment(date1, "month", -12, "start") = 01Aug2020;
Show(Date Increment(date2, "month", -12, "start")); //Date Increment(date2, "month", -12, "start") = 01Sep2020;
Show(Date Increment(Today(), "month", -12, "start"));
Here is a version to be used with data table with :date column:
Names Default To Here(1);
dt = Current Data Table();
dt << Select Where(:date >= Date Increment(Today(), "month", -12, "start"));
-Jarmo